An object at rest stays at rest, and an object in uniform motion stays in uniform motion, unless acted upon by a net external force. This defines inertia — the tendency of matter to resist changes in its state of motion. It also defines what an inertial reference frame is: one in which Newton's laws hold, i.e., a frame that is not itself accelerating.
Think carefully about what 'net force' means — objects in constant velocity have zero net force, not zero force. Everyday examples like a book on a table or a hockey puck sliding on ice make the concept tangible.
Before Newton, the dominant view of motion — tracing back to Aristotle — was that objects naturally come to rest, and that motion requires a continuous cause. Push a cart and it eventually stops; that seems to confirm that rest is the natural state and motion needs explaining. Newton's First Law overturns this picture completely. The question is not "what keeps things moving?" but "what changes their motion?" Inertia is the property of matter that resists changes in velocity — not changes in position, but changes in *how fast and in what direction* something is moving.
The law states: an object at rest stays at rest, and an object moving at constant velocity stays moving at constant velocity, unless a net external force acts on it. The word "net" is critical. From your kinematics work, you know that an object sitting on a table is not moving, but it is not experiencing zero force — gravity pulls it down and the normal force pushes it up, and these cancel. What matters is the vector sum of all forces. When net force is zero, velocity is constant (which includes zero as a special case). When net force is nonzero, velocity changes — that is the content of Newton's Second Law, which this topic builds toward.
The reason everyday experience misleads us is friction. When you push a book across a table and let go, it slows down — but not because motion naturally fades, because friction is a real force decelerating it. Imagine the same book on a frictionless surface in space: it would continue at constant velocity indefinitely. The hockey puck on nearly-frictionless ice approaches this ideal. What Aristotle saw as "natural rest" is actually the result of pervasive friction in everyday environments. Remove the friction, and the Newtonian picture is revealed: no force is required to *maintain* motion, only to *change* it.
Newton's First Law also defines the concept of an inertial reference frame — a coordinate system that is not itself accelerating. In a braking car, objects appear to fly forward "by themselves," apparently violating the First Law. But there is no force on those objects; it is the car (and your reference frame) that is decelerating. Newton's laws hold cleanly only from inertial frames. This is not a technicality but a foundational point: it sets the stage for all subsequent mechanics. Once you know what frame you are in and what forces are acting, the entire motion of a system is determined. The First Law tells you when no determination is needed — when things are simply left alone to continue doing what they are already doing.